1. Field of the Invention
The present invention relates to a method and an apparatus for cutting a cylindrical material and, in particular, to such method and apparatus in which a rotary blade is used to cut a cylindrical material (which is hereinafter referred to as an ingot) such as silicone or the like to produce semiconductor pieces.
2. Description of the Related Art
An ingot of silicone or the like used as a semiconductor device is fragile and, therefore, when the ingot is sliced into thin pieces with the rotary blade, the thin pieces being sliced may be cracked and thereby separated from the ingot at the cracked portion just before the slicing is completed. In other words, the sliced thin piece, which is generally referred to as a wafer, is chipped in the outer peripheral portion thereof, with the result that the number of semiconductor devices obtainable from a wafer may be greatly decreased.
In order to eliminate the above-mentioned disadvantage, there is employed a slicing method (for example, Japanese Patent Publication (Kokai) No. 61-65749), in which a sacrifice member (which is hereinafter called a slice base) of carbon or the like is attached to a part of the side surface of the ingot longitudinally thereof by means of adhesives or the like, the slicing begins at the side of the ingot opposite to the surface thereof to which the slice base is attached, and the slice base is cut lastly.
According to the above-mentioned slicing method, there can be eliminated the problem that the ingot and the wafer may be broken in part. But, however, the slicing method requires operations to previously prepare and attach the slice base and to remove the slice base from the sliced wafer and, in the slicing method, it is necessary to select a slice base of such a material which does not have ill effects on the cutting edge of the rotary blade.
On the other hand, there is also known another type of slicing method without attaching a slice base to an ingot, in which, while a blade is being rotated, the ingot is also rotated and sliced. (For example, Japanese Patent Publication (Kokai) No. 58-147312). When the ingot is rotated as in the last-mentioned method, since the ingot can be sliced from the periphery thereof toward the central portion thereof, there is eliminated the possibility that the outer periphery of the ingot, which provides a high productivity, may be broken, and the above-mentioned slice base is not necessary, which is advantageous over the formerly-mentioned prior art slicing method. However, from the viewpoint of performance of a slicing apparatus, a further higher accuracy and a further higher function is required.
In other words, when the wafer sliced is collected, according to the above-mentioned slice base attachment method, the wafer may be gripped or may be adsorbed and held by a vacuum chuck when the slice base is cut. On the other hand, according to the ingot rotation method, it is necessary to grip or absorb the wafer at least just before the slicing is completed. However, such absorption or gripping can cause the wafer to be vibrated or twisted, with the result that the wafer may be broken in the central portion thereof. Therefore, in order to solve this problem, it is necessary to rotate a gripping or adsorbing mechanism synchronously and coaxially with the ingot as well as to bring such mechanism into contact with the wafer in a skillful manner. In particular, as in the first slicing of the ingot, when the end face of the ingot is not flat or is not at right angles to the axis of rotation, it is difficult to rotate the ingot coaxially with the vacuum chuck. In this respect as well, it is difficult to realize and maintain the accuracy and function of the whole slicing apparatus. Also, in the ingot rotation method, there can be easily produced a projection as a so called "navel" in the central portion of the wafer when the wafer central portion is not broken. In this case, the projection must be removed and treated, but this after-treatment is very troublesome.